As some have said, he has to actually want help. What is even more important is that he has to be ready for help. I've seen this far too many times, with suicidal people, depressed people, and addicted people. They know there's something wrong with them, they don't like it, and they don't want to stay that way.
But, whatever got them there in the first place isn't done with them, and no matter how many times they cry out, they can't accept the help that is so commonly offered with the best of intentions. They all too often fall right back into the same destructive behavior that tears their friends and families apart.
Sometimes, and most definitely not every time, the best thing a friend can do is to talk to them and get them to the point where they are ready. This is incredibly difficult, because few of us are mental health professionals, because it's hard for them to believe you really understand, and because often it doesn't matter what anyone says.
If someone's not ready, it's nobody's fault. Not even theirs, although it can easily be their fault they got there in the first place. I believe the only reason anyone else should feel guilty is if their friend is ready for help, and you turn them away.